The present invention relates to a method for growing a nitride compound semiconductor.
Nitride compound semiconductors are used as materials for various types of semiconductor devices including blue-light-emitting semiconductor laser diodes. To improve the reliability and performance of a semiconductor device using a nitride compound semiconductor, a nitride compound semiconductor of excellent crystallinity must be grown on a substrate. In a prior art method, an AlN buffer layer is first grown on a sapphire substrate at a growth temperature in the range from 400xc2x0 C. to 900xc2x0 C., both inclusive, and then a nitride compound semiconductor is grown thereon (see Japanese Laid-Open Publication No. 2-22947, for example). In another prior art method, an AlGaN buffer layer is first grown on a sapphire substrate at a growth temperature in the range from 200xc2x0 C. to 900xc2x0 C., both inclusive, and then a nitride compound semiconductor is grown thereon (see Japanese Laid-Open Publication No. 7-312350 and Japanese Publication for opposition No. 8-8217, for example).
According to any of these methods, a polycrystalline buffer layer is grown on a sapphire substrate at a low temperature, which is then raised to change part of the buffer layer into single crystals. In that single crystalline part of the buffer layer, seed crystals with a satisfactorily aligned orientation are formed, and a nitride compound semiconductor is grown thereon using these seed crystals as nuclei of crystal growth. As the crystals grow, each of these nuclei grows larger and larger to impinge on an adjacent nucleus at last. In this case, since a pair of adjacent nuclei have an aligned orientation, these nuclei are combined to form one single crystal after the impingement. In this manner, nitride compound semiconductor crystals grow.
The present inventors analyzed in detail the cross section of nitride compound semiconductor crystals, which had been grown on a buffer layer formed on a sapphire substrate at a low temperature in accordance with the conventional method, using a transmission electron microscope. As a result, we found that although almost no defects were observed in the substrate, defects had developed from the interface between the substrate and the nitride compound semiconductor crystals in the buffer layer. Specifically, an enormous number of defects were observed in the nitride compound semiconductor crystals at a dislocation density in the range from about 1xc3x97109 cmxe2x88x922 to about 1xc3x971011 cmxe2x88x922. We believe that these defects are caused because when the growing nuclei are impinging on each other around that part of the buffer layer where single crystals have been formed, the crystal orientations and stacking heights thereof slightly deviate from each other at an atomic level and the nitride compound semiconductor crystals grow with these deviations left as crystal defects therein. If a semiconductor device is fabricated using these nitride compound semiconductor crystals where such defects exist, each of these defects functions as the non-radiative center at which carriers are trapped. As a result, the reliability of the device is seriously affected. For example, the emission efficiency decreases or the device is broken down because of these defects. Also, in growing nitride compound semiconductor crystals on a sapphire substrate, on and after the thickness of the grown layer reaches a critical value, the substrate is adversely cracked during a process of lowering the temperature of the substrate from a growth temperature to room temperature. This is due to a difference in thermal expansion coefficient between the substrate and the grown layer.
As can be understood, defects are almost always generated in a nitride compound semiconductor grown by the conventional method. Thus, an improved crystal growing method that can obviate such defects is required to further improve the performance and reliability of a semiconductor device using a nitride compound semiconductor.
An object of the present invention is providing a method for growing a nitride compound semiconductor on a substrate with these defects substantially obviated.
A method for growing a nitride compound semiconductor according to the present invention includes the step of growing a compound semiconductor expressed by a general formula AlxGa1-xN (where 0xe2x89xa6xc3x97xe2x89xa61) on a nitride compound semiconductor substrate at a temperature of about 900xc2x0 C. or more.
Another method for growing a nitride compound semiconductor according to the present invention includes the step of growing a compound semiconductor expressed by a general formula InyAlzGa1-y-zN (where 0 less than yxe2x89xa61, 0xe2x89xa6z less than 1 and 0xe2x89xa6y+zxe2x89xa61) on a nitride compound semiconductor substrate at a temperature of about 700xc2x0 C. or more.
In one embodiment of the present invention, a dislocation density in the substrate is preferably about 1xc3x97106 cmxe2x88x922 or less.
In another embodiment, the thickness of the substrate is preferably about 30 xcexcm or more.
In still another embodiment, either arsenic or a compound containing arsenic may be supplied onto the substrate while the compound semiconductor is being grown on the substrate.
In still another embodiment, either phosphorus or a compound containing phosphorus may be supplied onto the substrate while the compound semiconductor is being grown on the substrate.